1. Technical Field
The present disclosure is directed to an image frequency rejection mixer and to a mixing stage of an image frequency rejection mixer
2. Description of the Related Art
Electronic mixers for translating a radio frequency signal to an intermediate frequency are well known in the art. Such mixers are for example used in AM and FM radio receivers, wherein a received radio signal is translated from a carrier frequency, i.e., the radio frequency, to an intermediate frequency for signal processing, such as amplifying and pass-band filtering, before another translation into the base band takes place for outputting an audio signal to the user.
The basic idea of said frequency translation stems from a multiplication of two trigonometric signals, as illustrated in the following equation:VRF(t)*VLO(t)=V*RF cos(2πfRFt)*V*LO cos(2πfLOt)=½V*RFV*LO cos(2π(fRF−fLO)t)+½V*RFV*LO cos(2π(fRF+fLO)t)wherein fRF is the frequency of a received radio signal, fLO is the frequency of a local oscillation signal, and V*RF and V*LO are the amplitudes of the radio signal and the local oscillation signal, respectively. As is apparent from the first term of the right hand side of above equation, the intermediate frequency fIF is the difference between the desired radio frequency to be received fRF,desired and the local oscillation frequency. However, from basic trigonometric laws, it is known that cos(α)=cos(−α). Therefore, there exists an undesired radio frequency fRF,undesired that is mixed to the same intermediate frequency, with said undesired radio frequency satisfying the condition fRF,undesired−fLO=−fIF. This undesired radio frequency is also referred to as image frequency.
In order to solve this problem, complex mixers, so called image reject mixers, have been developed. These image reject mixers are comprised of two signal paths, with an input signal being multiplied with a first local oscillation signal in the first signal path and being multiplied with a second local oscillation signal in the second signal path. The first and the second local oscillation signals are in quadrature. For example, the two local oscillation signals may be a sin-signal and a cos-signal. With appropriate signal processing after the mixing operation, the two resulting mixed signals may be combined in a way that only the spectral component at the desired radio frequency is present in the mixer output intermediate frequency signal.
An example of a portion of such an image reject mixer is shown in FIG. 1. A mixer input signal is input at the image reject mixer inputs 10 and 12 of the mixing stage 1 of the image reject mixer. This differential input signal is amplified in the two transconductors 2 and 4, and the transconductor output signals are mixed in the mixing circuits 6 and 8, respectively. For these two mixing operations, a first local oscillation signal is supplied to the terminals 606 and 608, and a second local oscillation signal is supplied to the terminals 806 and 808. The mixing circuit output signals, present in differential form at the mixing circuit output terminals 618, 620 and 818, 820, are then post-processed in order to cancel the image frequency component at the mixer output.
Such a complex mixer has the advantage of rejecting the image frequency component of the received radio frequency signal. However, due to its complex layout, a lot of undesired electronic noise is generated. Such noise deteriorates the quality of the mixer output signal. Regarding the application of such a mixer to a radio receiver, the quality of the audio output presented to the user is reduced due to the electronic noise. It is equally valid to say that the electronic noise makes a higher signal-to-noise ratio at the receiver input necessary in order to still successfully recover the transmitted signal and output it to a user with a high quality.
Accordingly, it would be beneficial to provide an image frequency rejection mixer that reduces the electronic noise introduced in the signal processing path.